Neural tube closure defects are among the most common and severe congenital malformations. Before research efforts can be initiated toward elucidating the underlying causes of these tragic birth defects, it is essential to gain a better understanding of the mechanisms of neural tube formation. Although multiple factors (both intrinsic and extrinsic) may act in a concerted manner to shape a flat neuroepithelial sheet (the neural plate) into a hollow cylinder (the neural tube), the principal driving forces for this process are now thought to originate from changes in the shape of neuroepithelial cells (especially apical constriction) which presumably is a result of the contractile activity of apical microfilament bundles. However, very little is known about the organization and functional activity of these microfilament bundles or how forces resulting from their contractile activity are deployed and coordinated to bring about the complex sequence of morphogenetic movements that hallmark neural tube formation. While there is no method currently available to measure directly the contractile activity of apical microfilament bundles in neuroepithelial cells, our pilot studies on chick embryos have shown that morphometric measurements of the apical width (reflecting the degree of apical constriction) and the degree of apical surface folding of neuroepithelial cells can be used as a reliable barometer of local microfilament activity within the neuroepithelium. The proposed study will use day 7-9 mouse embryos as a model system to investigate further the role of microfilament-mediated cell shape changes in neural tube formation. Specifically, we will (1) use morphometric methods to determine the distribution of forces (resulting from microfilament-mediated apical constriction of neuroepithelial cells) within the neuroepithelium during formation of the major divisions of the neural tube (future forebrain, midbrain, hindbrain, and spinal cord), (2) further explore the role of apical microfilament bundles in mediating cell shape changes by investigating temporal changes in their structure and organization in neuroepithelial cells forming different regions of the neuroepithelium at various phases of neural tube formation, and (3) determine if the local intensity of forces is correlated with the distribution patterns of motility-related proteins in the developing neuroepithelium.